tinyprog: Pad to full minor sector write size

Either the TinyProg-BootLoader state machines, or the SPI flash chip
used on the TinyFPGA BX, seems unable to *properly* program a partial
minor sector (ie, writing less than 256 octets), leading to a failed
write/verify cycle.  This seems to happen at the end of any programming
that is not an exact multiple of 256 octets.  Since the whole 256 octets
is already erased in program_sectors(), we pad out the short write to
a full 256 octets (with 0xff, which is the value read for "erased", to
reduce wear on on the flash cells).

For more detail on diagnosing this, see:
timvideos/litex-buildenv#137

Author: ewenmcneill

programmer/tinyprog/__init__.py

@@ -499,6 +499,22 @@ def program_sectors(self, addr, data):
                 for minor_offset in range(0, 4 * 1024, minor_sector_size):
                     minor_write_data = current_write_data[
                         minor_offset:minor_offset + minor_sector_size]
+
+                    # The TinyFPGA firmware and/or flash chip does not handle
+                    # partial minor sector writes properly, so pad out short
+                    # writes to a whole minor sector.  (This is safe because
+                    # we explicitly erased the whole sector above, so the
+                    # following bytes must be freshly erased.  Usually it will
+                    # only happen on the final minor sector to be written.)
+                    #
+                    if (len(minor_write_data) < minor_sector_size):
+                         pad_len = minor_sector_size - len(minor_write_data)
+                         padding = b'\xff' * pad_len
+
+                         minor_write_data = bytearray(minor_write_data)
+                         minor_write_data.extend(padding)
+                         assert(len(minor_write_data) == minor_sector_size)
+
                     self.write(
                         current_addr + minor_offset,
                         minor_write_data,